yarn, fibers or filaments formed into a continuous strand for use in weaving textiles or for the manufacture of thread. A staple fiber, such as cotton, linen, or wool, is made into yarn by carding, combing (for fine, long staples only), drawing out into roving, then spinning. Continuous filaments, such as silk, rayon, and nylon, may be formed directly into yarn or may be cut into short lengths and prepared like staple fibers. Yarns are twisted to give them strength and smoothness; a clockwise twist is known as the Z twist and a counterclockwise twist is known as the S twist. Two or more strands twisted together form ply yarns. In slub yarns areas are left untwisted to vary the diameter for ornamental effects. Complex yarns, such as bouclé and ratiné, are made by twisting together yarns of different tensions or diameters. The relation between the weight of the raw fiber of staple yarns and the yarn length is expressed by the yarn number; the finer the yarn, the higher the number. In filament yarns the yarn number, expressed in deniers, increases with the coarseness of the yarn.